The present invention relates to the field of the measurement of electric currents and more particularly those having very high intensities carried by high voltage lines.
In certain environments, particularly in electric power stations or electrochemical planes, it is very difficult to use conventional measuring devices for measuring currents having high intensities and liable to vary considerably. Thus, in such environments, the high voltages, high temperatures, possibly corrosive atmosphere, electromagnetic pollution or difficult accessibility make measurements difficult and unreliable when carried out by conventional e.g. Hall effect gauge or shunt means. Thus, in the case of measuring currents of a few hundred thousand amperes, shunt-type current measuring devices only make it possible to obtain an accuracy of about 10% on the measured value and they also lead to energy losses in the measuring device.
For the purpose of improving such devices, it has been proposed to utilize the magnetooptical effect manifested, for example, in interferometric devices. As is shown, when a light wave propagates parallel to the lines of force of a magnetic field, the latter induces a non-reciprocal effect or Faraday effect on the wave.
U.S. patent application Ser. No. 171,285 of July 23rd 1980, now U.S. Pat. No. 4,370,612, relates to an optical fibre electric current measuring interferometer of this type. The device described in this patent application has an optical fibre wound around the conductor in which circulates the current to be measured, a laser source, means for separating the radiation emitted by the source and feeding it to the two ends of the wound fibre and for recombining the radiation emerging from these two ends, a conductor circuit traversed by a regulatable reference current, and a detector supplying a signal which is characteristic of the interferences between the two waves emerging from the fibre, the reference current being adjusted so that the phase shift between the two contrarotating waves in the fibre is zero, said current then being proportional to the current to be measured and circulating in the main conductor.
However, on measuring currents carried by high or very high voltage power lines, where the voltages reach 220 or even 400 kV, the insulation distances in air are respectively 2.5 and 4.5 m. Therefore it is not possible to merely surround the conductor carrying the current with one or more turns of the interferometer optical fibre and arrange in the vicinity thereof the electronic circuits supplying the regulatable reference current.
Other difficulties also occur. More specifically the aforementioned device acts as a current divider. The electronic circuits adjust the current in the feedback winding, so as to cancel out the effect of the current passing through the main loop on the phase of the light guided by the fibre. The two currents are then proportional to the number of turns in the windings only if the magnetooptical interaction coefficient is the same for both windings, one of them being reduced to its simplest expression, because it is a power transmission cable.
When these two windings are not superimposed, which is generally the case, it is not possible to apply Ampere's law, which guarantees this equality, is only possible if the polarization state of the light transferred is uniform along the closed path of the interferometer fibre.
No fibre perfectly retains the circular polarization state necessary for this operation. When, as is the case in very high voltage, it is necessary to move the feedback loop a considerable distance from the main current conductor in order to prevent a breakdown, the stability of the scaling factor can be considerably affected and the measurement is then falsified.